This invention relates generally to refrigeration and operation and more particularly to a method and apparatus for boosting the cooling capacity and efficiency of air-conditioning systems under a wide range of ambient atmospheric conditions.
In air conditioning, the basic circuit is essentially the same as in refrigeration. It comprises an evaporator, a condenser, an expansion valve, and a compressor. This, however, is where the similarity ends. The evaporator and condenser of an air conditioner will generally have less surface area. The temperature difference DT between condensing temperature and ambient temperature is usually 27.degree. F. with a 105.degree. F. minimum condensing temperature, while in refrigeration the difference DT can be from 8.degree. F. to 15.degree. F. with an 86.degree. F. minimum condensing temperature.
I have previously improved the cooling capacity and efficiency of refrigeration systems. As disclosed in my U.S. Pat. No. 4,599,873, this is accomplished by addition of a liquid pump at the outlet of the receiver or condenser. Operation of the pump adds 5-12 p.s.i. of pressure to the condensed refrigerant flowing into the expansion valve, a process I call liquid pressure amplification. This suppresses flash gas and assures a uniform flow of liquid refrigerant to the expansion valve, substantially increasing cooling capacity and efficiency. The best results are obtained when such a system is operated with the condenser at moderate ambient temperatures, usually under 80.degree. F. As ambient temperatures rise above the minimum condensing temperature, the advantages gradually decrease. The same thing happens when the principles of my prior invention are applied to air conditioning, except that the minimum condensing temperature is higher.
While conventional air-conditioning systems can benefit from my prior invention, the greatest need for air conditioning is when ambient temperatures are high, over 80.degree. F. Conventional air conditioning becomes less effective and efficient as ambient temperatures rise to 100.degree. F. or more, as does use of my prior liquid refrigerant pressure amplification technique.
I have since found that in large refrigeration or air conditioning systems, high refrigerant flow rates require multiple pumps in parallel or a larger single pump. The use of a larger single pump is often preferred for simplicity of design. In such systems the large electrically-driven compressors typically operate on a separate electrical circuit from the liquid pressure amplification pump motor. Should the power circuit to the liquid amplification pump motor be turned off or disconnected while the compressor motor circuit is still operable, the compressor will work to drive refrigerant through the pump. At high flow rates, the pressure drop through a centrifugal pump, ordinarily fitted with an output restrictor, will become higher than acceptable. In order to preserve all the available capacity of the partially disabled system under those circumstances, pressure drops in the system must be minimized where ever possible. Unfortunately, it is not possible to entirely eliminate the pressure drop through the idle liquid pressure amplification pump. If a positive displacement pump is used as the liquid pressure amplification pump, in place of the preferred centrifugal pump, the pump can block flow completely when its motor loses power. This, too, is unacceptable.
It is, therefore, an object of the invention to improve the efficiency of refrigeration and air-conditioning systems.
Another object of the invention is to increase the cooling capacity of such systems when operated at high ambient temperatures.
A further object of the invention is to enable the aforementioned objects to be attained economically and by retrofitting existing systems as well as in new systems.
A third object of the invention is to minimize the pressure drop imposed on the operating refrigeration or air conditioning system by the liquid pressure amplification pump when idle.
The present invention is an improvement in the structure and method of operation of an air-conditioning or refrigeration system which includes a compressor, a condenser, an expansion valve, an evaporator, and conduit means interconnecting the compressor, condenser, expansion valve and evaporator in series in a closed loop for circulating refrigerant therethrough, and optionally may include a receiver between the condenser and expansion valve. The conduit means includes first conduit means coupling an outlet of the compressor to an inlet to the condenser to convey superheated vapor refrigerant from the compressor into the condenser at a first pressure and temperature. A liquid pump means has an inlet coupled to an outlet of the condenser (or to the receiver outlet) for receiving condensed liquid refrigerant at a second pressure less than said first pressure and boosting the second pressure of the condensed liquid refrigerant by a substantially constant increment of pressure within a predetermined range to discharge the condensed liquid refrigerant in a forward direction from an outlet of the pump means at a third pressure greater than said second pressure. A second conduit means couples the outlet of the pump means to an inlet to the expansion valve to transmit a first portion of the condensed liquid refrigerant from outlet of the pump means at said third pressure through the expansion valve into the evaporator to vaporize and effect cooling for air conditioning or refrigeration. A third conduit means couples the outlet of the pump means to an inlet to the condenser to transmit a second portion of the condensed liquid refrigerant from outlet of the pump means into the inlet of the condenser to vaporize therein. The portion of the condensed liquid refrigerant injected into the condenser inlet cools the superheated vapor refrigerant entering the condenser to a reduced temperature, thereby reducing said first pressure.
The first and second conduit means are preferably proportioned so that the second portion of refrigerant is sufficient to reduce the first temperature to a reduced temperature close to a saturation temperature of the refrigerant, preferably within 10.degree. F. to 15.degree. F. above saturation temperature, and so that the second portion of refrigerant is substantially less than the first portion, preferably less than about 5% of the first portion and typically in the range of 2%-3% of the first portion. Suitably, the first and second conduit means are proportioned with a cross-sectional area ratio of about 16:1. The system preferably further includes means responsive to a temperature of the evaporator for modulating the expansion valve.
The system further includes a bypass conduit connected between the intake and outlet of the liquid pressure amplification pump, and a flow control means in the bypass conduit, through which refrigerant flows in the forward direction responsive to a predetermined pressure differential, and which blocks refrigerant flow in a reverse direction responsive to a reversal of the pressure differential. The flow control means preferably includes a check valve, or can include an electrically operated solenoid valve.
In the improved method of operation, superheated vapor refrigerant is transmitted from the compressor to an inlet to the condenser at a first temperature and pressure. The vapor refrigerant is condensed and discharged as liquid refrigerant at a second temperature and pressure less than said first temperature and pressure. The pressure of the liquid refrigerant discharged from the condenser (or receiver) is boosted to a third pressure greater than the second pressure by a substantially constant increment of pressure. Then, in accordance with the invention, a first portion of the liquid refrigerant is transmitted at said third pressure via the expansion valve into the evaporator and a second portion thereof is transmitted into the condenser inlet so that the first temperature of the superheated vapor refrigerant is reduced toward said second temperature, thereby reducing said first pressure.
The first and second portions of liquid refrigerant at said third pressure are proportioned so that the first portion is substantially greater than the second portion. Preferably, the added increment of pressure is 8 to 10 p.s.i. and the second portion has a flow rate less than 5% of the flow rate of the first portion. The flow of the first portion through the expansion valve can be modulated in response to a temperature in the evaporator.
Prior art ammonia-refrigeration systems are known in which a portion of liquid refrigerant is injected from the receiver to the condenser inlet to suppress superheat. This has not been done, however, in combination with adding an incremental pressure, for example by means of a centrifugal pump, to the pressure of the liquid refrigerant flowing into the expansion valve.
Operation with an added incremental liquid refrigerant pressure preferably includes allowing the first pressure to float with an ambient temperature. This reduces overall system pressures, thereby increasing system efficiency at moderate ambient temperatures. The present invention desuperheats the compressed refrigerant vapor as it enters the condenser, lowering its temperature and further reducing the first pressure, even when ambient temperatures are high. The invention thus raises the temperature range over which benefits can be obtained from adding an increment of pressure to the liquid refrigerant. This further improves efficiency and enables effective operation in very high ambient temperature environments.
The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings.